Superconducting cable line

ABSTRACT

A superconducting cable line includes a heat insulation pipe for a fluid for transporting liquid hydrogen, a superconducting cable housed in the heat insulation pipe for a fluid, and heat exchange means for performing a heat exchange between liquid hydrogen and a refrigerant of the cable. The superconducting cable includes a cable core inside a heat insulation pipe for a cable and is housed in the heat insulation pipe for a fluid to form a low temperature environment around the cable and a double heat insulation structure including the heat insulation pipe. Therefore, since heat intrusion into the superconducting cable is reduced and the refrigerant is cooled with liquid hydrogen, the line can reduce energy for cooling the refrigerant.

TECHNICAL FIELD

The present invention relates to a line for power supply including asuperconducting cable. More specifically, the present invention relatesto a superconducting cable line which decreases heat intrusion into asuperconducting cable to reduce energy for cooling a refrigerant used inthe cable and can increase a coefficient of performance (COP) as a wholeline.

BACKGROUND ART

A superconducting cable including a heat insulation pipe housing a cablecore having a superconducting conductor layer has been conventionallyknown. Such a superconducting cable includes, for example, a single-corecable having a heat insulation pipe housing one cable core or athree-core cable housing three cable cores in a bundle. FIG. 7 is across-sectional view of a three-core superconducting cable forthree-phase AC transmission. FIG. 8 is a cross-sectional view of eachcable core 102. This superconducting cable 100 has a construction inwhich three stranded cable cores 102 are housed in a heat insulationpipe 101. Heat insulation pipe 101 has a construction in which a heatinsulating material (not shown) is arranged between a double pipe formedwith an external pipe 101 a and an internal pipe 101 b, and air betweenthe pipes 101 a, 101 b is evacuated. Each cable core 102 includes, froma center portion thereof, a former 200, a superconducting conductorlayer 201, an electrical insulation layer 202, a superconducting shieldlayer 203, and a protection layer 204. Space 103 enclosed with internalpipe 101 b and each cable core 102 becomes a passage of a refrigerantsuch as liquid nitrogen. A superconducting state of superconductingconductor layer 201 and superconducting shield layer 203 of cable core102 is maintained by cooling with the refrigerant. A corrosion-prooflayer 104 is included on an outer periphery of heat insulation pipe 101.

The superconducting cable must be continuously cooled with therefrigerant such as liquid nitrogen to maintain the superconductingstate of the superconducting conductor layer and the superconductingshield layer. Therefore, a line using the superconducting cable usuallyincludes a cooling system for a refrigerant. With this system,circulation cooling is performed in which the refrigerant ejected fromthe cable is cooled and the cooled refrigerant again flows into thecable.

With cooling of the refrigerant to an appropriate temperature by thecooling system, the superconducting cable can maintain thesuperconducting state of the superconducting conductor layer and thesuperconducting shield layer by sufficiently decreasing an increase in atemperature of the refrigerant due to heat generated by passage of acurrent or heat intrusion into the cable from the outside such as anatmosphere. When the refrigerant is liquid nitrogen, however, energyrequired for cooling the refrigerant to address such generated heat orheat intrusion becomes at least 10 times higher than energy handled bythe refrigerant to cool the cable. Therefore, when the superconductingcable line including the cooling system for the refrigerant isconsidered as a whole, a coefficient of performance (COP) becomes about0.1 or lower. Such a low COP is one of causes of a decreased applicationeffect of a superconducting apparatus such as a superconducting cable.Thus, each of Japanese Patent Laying-Open No. 2002-130851 (PatentDocument 1) and Japanese Patent Laying-Open No. 10-092627 (PatentDocument 2) proposes to cool a refrigerant of a superconducting coilutilizing cold heat of a liquefied natural gas (an LNG).

On the other hand, with proceeding development of a fuel cell vehicle,there are plans to build hydrogen stations at many places in Japan forstoring compressed hydrogen or liquid hydrogen to be fed to the fuelcell vehicle. The hydrogen station includes, for example, a tank forstoring liquid hydrogen produced in a factory and transported or liquidhydrogen produced in the station, and a cooling system for liquefyingvaporized hydrogen to maintain a liquid state. Though hydrogen can bemaintained in the liquid state by cooling to an appropriate temperaturewith this cooling system, heat intrusion into the cable from the outsidebecomes large since liquid hydrogen has a cryogenic boiling point ofabout 20 K which is substantially different from an ordinary temperatureof an atmosphere. Therefore, an enormous amount of energy is requiredfor cooling liquid hydrogen to reduce an increase in a temperature dueto heat intrusion.

Patent Document 1: Japanese Patent Laying-Open No. 2002-130851

Patent Document 2: Japanese Patent Laying-Open No. 10-092627

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

Each of Patent Documents 1 and 2 described above merely disclosesutilization of cold heat of an LNG for cooling a refrigerant of asuperconducting coil, and does not consider as to reduction of heatintrusion from the outside. On the other hand, in a liquid hydrogenstation, it is also desired to decrease energy for cooling hydrogen, asdescribed above.

Therefore, a main object of the present invention is to provide asuperconducting cable line which can reduce heat intrusion into thesuperconducting cable and can totally reduce energy for cooling thesuperconducting cable and energy for cooling liquid hydrogen.

Means for Solving the Problems

The present invention attains the above-described object by arranging asuperconducting cable in a heat insulation pipe transporting liquidhydrogen and exchanging heat between liquid hydrogen and a refrigerantof the cable. That is, a superconducting cable line of the presentinvention includes a heat insulation pipe for a fluid for transportingliquid hydrogen and a superconducting cable housed in the heatinsulation pipe for a fluid for cooling a superconducting portion with arefrigerant having a temperature higher than that of liquid hydrogen.Heat exchange means for cooling liquid hydrogen and rising a temperatureof the refrigerant of the superconducting cable cooled with liquidhydrogen is further included. The present invention will be described ina more detail in the following.

The superconducting cable utilized in the present invention has aconstruction including a superconducting portion formed with asuperconducting material and a heat insulation pipe (hereafter referredto as a heat insulation pipe for a cable) housing the superconductingportion and filled with a refrigerant for cooling the superconductingportion. The superconducting portion may include a superconductingconductor layer for passing a current for power supply and an externalsuperconducting layer for passing a current having substantially thesame value as that for the superconducting conductor layer in anopposite direction. The superconducting portion is generally formed in acable core. Therefore, the superconducting cable may be constructed byhousing the cable core including the superconducting layer in the heatinsulation pipe for a cable. A more specific construction of the cablecore may include, from a center portion thereof, a former, asuperconducting conductor layer, an electrical insulation layer, anexternal superconducting layer, and a protection layer. The heatinsulation pipe for a cable may house one cable core (a single core (onecore)) or a plurality of cable cores (a plurality of cores). Morespecifically, when the line of the present invention is used forthree-phase AC transmission, for example, a three-core cable having theheat insulation pipe for a cable housing three stranded cores may beutilized, and when the line of the present invention is used forsingle-phase AC transmission, a single-core cable having the heatinsulation pipe for a cable housing one core may be utilized. When theline of the present invention is used for DC transmission (monopolartransmission), for example, a single-core cable having the heatinsulation pipe for a cable housing one core may be utilized, and whenthe line of the present invention is used for DC transmission (bipolartransmission), a two-core cable or a three-core cable having the heatinsulation pipe for a cable housing two or three stranded cores may beutilized. As described above, the superconducting cable line of thepresent invention can be utilized for either of the DC transmission andthe AC transmission.

The superconducting conductor layer may be formed by, for example,spirally winding a tape-like wire including a plurality of filamentsmade of a Bi-based oxide superconducting material, more specifically, aBi2223-based superconducting material which are arranged in a matrixsuch as a silver sheath. The superconducting conductor layer may have asingle-layer or multilayer structure. When the superconducting conductorlayer has a multilayer structure, an interlayer insulation layer may beprovided therein. The interlayer insulation layer may be provided bywinding insulating paper such as kraft paper or semisynthetic insulatingpaper such as PPLP (a trademark of Sumitomo Electric Industries, Ltd.).The superconducting conductor layer is formed by winding a wire made ofthe superconducting material around the former. The former may be asolid or hollow body formed with a metal material such as copper oraluminum, and may have a construction of, for example, a plurality ofstranded copper wires. A copper wire having insulating coating may beutilized. The former functions as a shape-maintaining member of thesuperconducting conductor layer. A cushion layer may be interposedbetween the former and the superconducting conductor layer. The cushionlayer avoids direct contact of metals between the former and asuperconducting wire to prevent the superconducting wire from beingdamaged. In particular, when the former has a stranded structure, thecushion layer also has a function to smooth a surface of the former.Insulating paper or carbon paper can be suitably utilized as a specificmaterial of the cushion layer.

The electrical insulation layer may be formed by winding semisyntheticinsulating paper such as PPLP (trademark) or insulating paper such askraft paper on the superconducting conductor layer. A semi-conductinglayer may be formed with carbon paper or the like on at least one of aninner periphery and an outer periphery of the electrical insulationlayer, that is, between the superconducting conductor layer and theelectrical insulation layer and between the electrical insulation layerand the external superconducting layer (described below). With formationof an internal semi-conducting layer, the former, or an externalsemi-conducting layer, the latter, adhesion between the superconductingconductor layer and the electrical insulation layer or between theelectrical insulation layer and the external superconducting layer isincreased to suppress deterioration due to an occurrence of partialdischarge or the like.

When the line of the present invention is used for DC transmission, theelectrical insulation layer may be subjected to ρ grading for attaininga low resistivity on an inner periphery side of the electricalinsulation layer and a high resistivity on an outer periphery side tosmooth a DC electric field distribution in a diameter direction (athickness direction) thereof. As described above, “ρ grading” meansvarying of a resistivity in the thickness direction of the electricalinsulation layer in a stepwise manner, which can smooth the DC electricfield distribution in a whole thickness direction of the electricalinsulation layer and can decrease a thickness of the electricalinsulation layer. Though a number of layers having varied resistivitiesis not specifically limited, two or three layers are practical. Inparticular, smoothing of the DC electric field distribution can beperformed more effectively when a thickness of each layer is equalized.

The ρ grading may be performed using insulating materials havingresistivities (ρ) different from each other. When insulating paper suchas kraft paper is utilized, for example, a resistivity. can be varied byvarying a density of the kraft paper or adding dicyandiamide to thekraft paper. When composite paper formed with insulating paper and aplastic film such as PPLP (trademark) is utilized, a resistivity can bevaried by varying a ratio k=(tp/T)×100, a ratio of a thickness tp of theplastic film to a thickness T of whole composite paper, or by varying adensity, a material, an additive or the like of the insulating paper. Avalue of ratio k is preferably within a range of, for example, about40-90%. Generally, resistivity ρ becomes higher as ratio k increases.

Furthermore, when the electrical insulation layer has a high ε layerprovided near the superconducting conductor layer and having apermittivity higher than that in another portion, an Imp. withstandvoltage property can be increased in addition to an increase in a DCwithstand voltage property. A permittivity ε (20° C.) is about 3.2-4.5in general kraft paper, about 2.8 in composite paper having ratio k of40%, about 2.6 in composite paper having the ratio of 60%, and about 2.4in composite paper having the ratio 80%. The electrical insulation layerconstructed with composite paper using kraft paper having high ratio kand higher airtightness is especially preferable because both of a DCwithstand voltage and an Imp. withstand voltage are increased.

A cable also suitable for AC transmission is formed by constructing theelectrical insulation layer to have permittivity E increased toward theinner periphery side and decreased toward the outer periphery side, inaddition to the ρ grading described above. This “ε grading” is alsoperformed over a whole region in the diameter direction of theelectrical insulation layer. In addition, the superconducting cablesubjected to the p grading described above has good DC characteristicsand can be suitably utilized as a DC transmission line. On the otherhand, most of current transmission lines are constructed for ACtransmission. When a transmission system is shifted from an AC system toa DC system, a situation may occur in which an AC is transientlytransmitted using the superconducting cable subjected to the ρ gradingbefore shifting to the DC transmission. This situation may occur when,for example, a cable of a portion of a transmission line was replacedwith the superconducting cable subjected to the ρ grading but the otherportions are still a cable for the AC transmission, or when the cablefor the AC transmission of the transmission line was replaced with thesuperconducting cable subjected to the ρ grading but a transmissionapparatus connected to the cable is still an apparatus for the AC. Inthis situation, the AC transmission is transiently performed with thesuperconducting cable subjected to the ρ grading, and then the system isfinally shifted to the DC transmission. Therefore, the superconductingcable is preferably designed not only to have the good DCcharacteristics but also with AC characteristics taken intoconsideration. When the AC characteristics are also taken intoconsideration, the superconducting cable having a good impulsecharacteristic such as a surge can be constructed by constructing theelectrical insulation layer to have permittivity ε increased toward theinner periphery side and decreased toward the outer periphery side.Then, when a transient period as described above is ended and the DCtransmission is performed, the superconducting cable subjected to the ρgrading used in the transient period can be continuously utilized as aDC cable. That is, the line using the superconducting cable subjected tothe ε grading in addition to the ρ grading can be suitably utilized foreach of the DC transmission and the AC transmission, and can also besuitably utilized as a line for both of AC and DC transmission.

PPLP (trademark) described above generally has a higher ρ value and alower ε value as ratio k is increased. Therefore, ρ can be increasedtoward the outer periphery side and, concurrently, ε can be decreasedtoward the outer periphery side when the electrical insulation layer isconstructed using PPLP (trademark) having ratio k increased toward theouter periphery side of the electrical insulation layer.

On the other hand, kraft paper generally has a higher ρ value and ahigher ε value as airtightness is increased. Therefore, it is difficultto construct the electrical insulation layer having ρ increased towardthe outer periphery side and ε decreased toward the outer periphery sideonly with kraft paper. Therefore, the electrical insulation layer issuitably constructed using kraft paper in combination with compositepaper. As an example, a kraft paper layer may be formed on the innerperiphery side of the electrical insulation layer and a PPLP layer maybe formed on the outside thereof to make resistivity ρ lower in thekraft paper layer than in the PPLP layer and permittivity ε higher inthe kraft paper layer than in the PPLP layer.

The external superconducting layer is provided on the outer periphery ofthe electrical insulation layer described above. The externalsuperconducting layer is formed with a superconducting material, as forthe superconducting conductor layer. The superconducting materialsimilar to that utilized to form the superconducting conductor layer maybe used in the external superconducting layer. When the superconductingcable line of the present invention is utilized for the DC transmission,the external superconducting layer may be utilized as, for example, areturn conductor in monopolar transmission or a neutral conductor layerin bipolar transmission. In particular, when the bipolar transmission isperformed, the external superconducting layer may be utilized to pass anunbalanced current when an unbalance occurs between a positive electrodeand a negative electrode. In addition, when one electrode is in anabnormal state and the bipolar transmission is changed to the monopolartransmission, the external superconducting layer may be utilized as areturn conductor for passing a current equivalent to a transmittedcurrent flowing through the superconducting conductor layer. When thesuperconducting cable line of the present invention is utilized for theAC transmission, the external superconducting layer may be utilized as ashield layer passing a shield current induced by a current flowingthrough the superconducting conductor layer. A protection layer also forinsulation may be provided on an outer periphery of the externalsuperconducting layer.

The heat insulation pipe for a cable for housing the cable core having aconstruction as described above may have a double pipe structure formedwith an external pipe and an internal pipe, which includes a heatinsulating material between the pipes and evacuation is performed toattain a prescribed degree of vacuum to form a vacuum insulationconstruction. Space inside the internal pipe is utilized as arefrigerant passage which is filled with a refrigerant such as liquidnitrogen for cooling the cable core (especially, the superconductingconductor layer and the external superconducting layer). The heatinsulation pipe for a cable as such is preferably a flexible corrugatedpipe. In particular, the heat insulation pipe for a cable is preferablyformed by a metal material such as stainless having high strength.

The refrigerant filling the heat insulation pipe for a cable which isutilized in the present invention has a temperature higher than that ofliquid hydrogen transported inside the heat insulation pipe for a fluid.Liquid nitrogen, for example, is utilized as the refrigerant. Sinceliquid hydrogen has a temperature lower than that of the refrigerant ofthe superconducting cable, the refrigerant of the superconducting cablehoused in the heat insulation pipe for a fluid is cooled with liquidhydrogen. Therefore, in the line of the present invention, a temperaturecapable of maintaining a superconducting state of the superconductingportion can be set without providing a separate cooling system for therefrigerant for cooling the refrigerant of the superconducting cable.

In the line of the present invention, the superconducting cable havingthe heat insulation pipe for a cable is housed in the heat insulationpipe for a fluid used for transportation of liquid hydrogen. With thisconstruction, the superconducting cable housed in the heat insulationpipe for a fluid has an environment around the cable having atemperature lower than an ordinary temperature, more specifically, acryogenic environment of about 20 K which is the temperature of liquidhydrogen, and thus a temperature difference between the inside and theoutside of the heat insulation pipe for a cable is decreased to lessthan 200 K as compared to a situation of laying in an atmosphere. Inparticular, when liquid nitrogen is used as the refrigerant of thecable, the temperature difference between the inside and the outside ofthe heat insulation pipe for a cable becomes about 50 K. In addition,the superconducting cable housed in the heat insulation pipe for a fluidhas a double heat insulation structure formed with a heat insulationstructure for liquid hydrogen and a heat insulation structure of thecable itself. Therefore, since the line of the present invention has asmall temperature difference between the inside and the outside of theheat insulation pipe for a cable and the superconducting cable havingthe double heat insulation structure as described above, heat intrusionfrom the outside into a cable portion can be effectively reduced ascompared to a superconducting cable line laid in the atmosphere.

A heat insulation pipe having heat insulation performance correspondingto liquid hydrogen transported therein may be utilized as the heatinsulation pipe for a fluid housing the superconducting cable. As anexample, a heat insulation pipe having a construction similar to thatfor the superconducting cable may be utilized, that is, a constructionhaving a double pipe structure formed with an external pipe and aninternal pipe, which includes a heat insulating material between thepipes and is subjected to evacuation. In this situation, space insidethe internal pipe becomes a transportation passage for liquid hydrogen.

When the heat insulation pipe for a fluid is formed by welding a metalplate made of stainless, steel or the like, for example, thesuperconducting cable may be housed in the heat insulation pipe for afluid by arranging the cable on the plate, bending the plate to coverthe cable, and welding edges of the plate. When a metal tube made ofstainless, steel or the like is used as the heat insulation pipe for afluid, the cable can be housed in the heat insulation pipe for a fluidby inserting the superconducting cable into the tube. In this situation,a skid wire (a slide wire) may be spirally wound around the cable toimprove an insertion property of the superconducting cable. Inparticular, when the heat insulation pipe for a cable is a corrugatedpipe having projections and depressions, the insertion property isimproved by winding the skid wire with a pitch larger than that of theprojections and depressions of the corrugated pipe (a long pitch) toprevent the skid wire from entering a depressed portion of thecorrugated pipe to locate the skid wire over the projections anddepressions to prevent an outer periphery of the corrugated pipe fromdirectly contacting the heat insulation pipe for a fluid, that is, toachieve point contact between the skid wire wound around the corrugatedpipe and the heat insulation pipe for a fluid. Furthermore, a tensionmember or the like may be attached to the superconducting cable to drawinto the heat insulation pipe for a fluid.

The superconducting cable housed in the heat insulation pipe for a fluidmay be arranged to contact liquid hydrogen transported inside the heatinsulation pipe for a fluid or not to contact liquid hydrogen. In theformer situation, the superconducting cable may be immersed in liquidhydrogen. In this situation, since a whole periphery of thesuperconducting cable contacts cryogenic liquid hydrogen, heat intrusionfrom the outside into the cable can be effectively reduced and therefrigerant of the cable can be sufficiently cooled with liquidhydrogen.

When the superconducting cable is immersed in liquid hydrogen, on theother hand, a problem such as an explosion of liquid hydrogen may arisein case the superconducting cable is short-circuited, for example, togenerate a spark. Therefore, a region inside the heat insulation pipefor a fluid may be divided into a transportation region for liquidhydrogen and a region for arranging the superconducting cable therein.As the transportation region, for example, a transportation pipe forliquid hydrogen may be separately arranged inside the heat insulationpipe for a fluid, and the superconducting cable may be arrangedlongitudinally along the transportation pipe. In this situation, when aheat exchanger spacer having high heat conductivity is arranged in spaceinside the heat insulation pipe for a fluid not occupied by thetransportation pipe and the superconducting cable, heat from liquidhydrogen can be efficiently conducted to the cable via the heatexchanger spacer, and therefore the cable can be cooled effectively. Theheat exchanger spacer as such may be formed with, for example, amaterial having high heat conductivity such as aluminum. Morespecifically, the heat exchanger spacer may be formed by windingaluminum foil.

In the present invention, the superconducting cable utilizing therefrigerant having a temperature higher than that of liquid hydrogen isused and, since the cable is housed in the heat insulation pipe for afluid which transports liquid hydrogen, the refrigerant can be cooledwith liquid hydrogen. The refrigerant of the superconducting cable,however, may be excessively cooled with liquid hydrogen andsolidification of the refrigerant may occur. Therefore, it is desirableto rise a temperature of the excessively cooled refrigerant of thesuperconducting cable housed in the heat insulation pipe for a fluidwithin a temperature range capable of maintaining the superconductingstate. On the other hand, liquid hydrogen is desirably cooled tomaintain a liquid state (to liquefy). Therefore, the present inventionincludes heat exchange means for exchanging heat between liquid hydrogenand liquid nitrogen in order to cool liquid hydrogen and rise atemperature of the refrigerant excessively cooled with liquid hydrogen.

The heat exchange means may have a construction including, for example,a passage circulating a heat-exchanging medium, an expansion valveexpanding the heat-exchanging medium, a compressor compressing theexpanded heat-exchanging medium, and a heat insulation case housing thepassage, the expansion valve and the compressor. A transportationpipeline for liquid hydrogen is arranged on a portion of the passagewhich passed through the expansion valve so as to cool liquid hydrogenwith the expanded heat-exchanging medium, while a transportationpipeline for the refrigerant of the cable is arranged on a portion ofthe passage which passed through the compressor so as to rise atemperature of the refrigerant of the superconducting cable with thecompressed heat-exchanging medium. The transportation pipeline forliquid hydrogen may be provided to form, for example, a circulation pathin which liquid hydrogen ejected from the heat insulation pipe for afluid again flows into the heat insulation pipe for a fluid.Alternatively, a tank storing liquid hydrogen may be connected to theheat insulation pipe for a fluid, and the transportation pipeline may beprovided to form a circulation path in which liquid hydrogen ejectedfrom the tank again flows into the tank. Then, a portion of thetransportation pipeline for liquid hydrogen as such is arranged tocontact the portion of the passage of the heat-exchanging medium whichpassed through the expansion valve, or arranged adjacent to the portion.The transportation pipeline for the refrigerant may be provided to forma circulation path in which the refrigerant ejected from the heatinsulation pipe for a cable again flows into the heat insulation pipefor a cable. Then, a portion of the transportation pipeline for therefrigerant as such is arranged to contact the portion of the passage ofthe heat-exchanging medium which passed through the compressor, orarranged adjacent to the portion. In the heat exchange means, thetemperature of the refrigerant is risen within a temperature rangecapable of maintaining the superconducting state of the superconductingportion. Since the heat exchange means for cooling liquid hydrogen andconcurrently heating the refrigerant of the cable is included, thepresent invention can concurrently meet both of a requirement oftemperature rising of the refrigerant of the superconducting cable and arequirement of cooling of liquid hydrogen.

It is to be noted that, in the present invention, since heat intrusioninto the superconducting cable housed in the heat insulation pipe for afluid is reduced as described above, a heat insulation structure of theheat insulation pipe for a cable can be simplified, that is, a level ofheat insulation performance for the heat intrusion from the outside intothe cable can be made lower. When the heat insulation pipe for a cablehas a construction of a double pipe structure formed with an externalpipe and an internal pipe, in which a heat insulating material isarranged between the pipes and evacuation is performed, the heatinsulation performance can be varied by, for example, varying a degreeof vacuum between the external pipe and the internal pipe, varying anumber of winding of the heat insulating material arranged between theexternal pipe and the internal pipe, or varying a material of the heatinsulating material.

In addition, in the superconducting cable line of the present invention,a whole length in a longitudinal direction of the superconducting cableforming the line may be housed in the heat insulation pipe for a fluid,or only a portion of the cable may be housed in the heat insulation pipefor a fluid. Considering reduction of heat intrusion, it is preferableto house a whole length of the superconducting cable in the heatinsulation pipe for a fluid.

The superconducting cable line of the present invention as such may beconstructed by, for example, housing the superconducting cable in apipeline having a heat insulation structure for connecting a hydrogenplant producing liquid hydrogen with a hydrogen station storing liquidhydrogen, or in a heat insulation pipe transporting liquid hydrogen inthe hydrogen station, and providing the heat exchange means near thehydrogen station. The line of the present invention may be utilized tosupply power to various power apparatuses used in the hydrogen station,or to draw power from the pipeline as required to supply power to eachplace.

As described above, the superconducting cable line of the presentinvention can be utilized for either of the DC transmission and the ACtransmission. When three-phase AC transmission is performed, forexample, the cable may be formed as a three-core superconducting cable,in which the superconducting conductor layer of each core is utilizedfor transmission of each phase and the external superconducting layer ofeach core is utilized as a shield layer. When single-phase ACtransmission is performed, the cable may be formed as a single-coresuperconducting cable, in which the superconducting conductor layerincluded in the core is utilized for transmission of the phase and theexternal superconducting layer is utilized as a shield layer. Whenmonopolar DC transmission is performed, the cable may be formed as asingle-core superconducting cable, in which the superconductingconductor layer of the core is utilized as a go conductor and theexternal superconducting layer is utilized as a return conductor. Whenbipolar DC transmission is performed, the cable may be formed as atwo-core superconducting cable, in which the superconducting conductorlayer of one core is utilized for positive electrode transmission, thesuperconducting conductor layer of the other core is utilized fornegative electrode transmission, and the external superconducting layerof each core is utilized as a neutral conductor layer.

In addition, the superconducting cable line of the present invention canalso be utilized as a line for both of the DC and AC transmission byutilizing the superconducting cable including the cable core having theelectrical insulation layer subjected to ρ grading and ε grading asdescribed above. In this situation, not only the superconducting cablebut also a terminal structure formed in an end portion of the line forconnecting the superconducting cable with a conductive portion on a sideof an ordinary temperature (a normal-conducting cable, a lead connectedto the normal-conducting cable or the like) is preferably constructed tobe suitable for both of the DC and AC transmission. A representativeconstruction of the terminal structure includes an end portion of thecable core extending from an end portion of the superconducting cable,an extraction conductor portion connected to the conductive portion onan ordinary temperature side, a connection portion electricallyconnecting the end portion of the core with the extraction conductorportion, and an end connection box housing the end portion of the core,an end portion of the extraction conductor portion on a side connectedto the core, and the connection portion. The end connection boxgenerally includes a refrigerant bath cooling the end portion of thecore or the end portion of the extraction conductor portion, and avacuum insulation bath arranged on an outer periphery of the refrigerantbath. In the terminal structure as such, a cross-sectional area of aconductor of the extraction conductor portion is desirably variablebecause an amount of a current flowing through the extraction conductorportion may be different in the AC transmission and the DC transmission.Therefore, a suitable construction of the terminal structure for both ofthe AC and DC transmission has a cross-sectional area of the conductorof the extraction conductor portion variable according to a load. Theterminal structure as such may have a construction, for example, inwhich the extraction conductor portion is divided into a lowtemperature-side conductor portion connected to the end portion of thecore and an ordinary temperature-side conductor portion arranged on aside of the conductive portion on the ordinary temperature side, whichlow temperature-side conductor portion and ordinary temperature-sideconductor portion are removable from each other. Furthermore, aplurality of removable extraction conductor portions as such areincluded to allow the cross-sectional area of the conductor of a wholeextraction conductor portion to vary according to a number ofconnections between the low temperature-side conductor portions and theordinary temperature-side conductor portions. A cross-sectional area ofthe conductor of each extraction conductor portion may be the same ordifferent from each other. The superconducting cable line of the presentinvention including the terminal structure as such can readily changefrom the DC transmission to the AC transmission, or from the ACtransmission to the DC transmission, by performing attachment or removalof the extraction conductor portion. In addition, since thecross-sectional area of the conductor of the extraction conductorportion can be varied as described above, the cross-sectional area ofthe conductor can also be varied as appropriate when an amount ofsupplied power is varied during the AC transmission or the DCtransmission.

Effects of the Invention

In a superconducting cable line according to the present inventionhaving a construction as described above, a superconducting cable ishoused in a heat insulation pipe transporting liquid hydrogen todecrease a temperature difference between the inside and outside of aheat insulation pipe for a cable, and a heat insulation structure of thecable is formed as a double heat insulation structure including the heatinsulation pipe for a cable and a heat insulation pipe for a fluid toeffectively reduce heat intrusion into the cable. In addition, in theline of the present invention, a refrigerant of the superconductingcable can be cooled with liquid hydrogen transported in the heatinsulation pipe for a fluid. With reduction of heat intrusion andcooling of the refrigerant utilizing a fluid as described above, theline of the present invention can substantially decrease orsubstantially eliminate energy for cooling the refrigerant of the cable.In particular, a cooling system for the refrigerant of thesuperconducting cable is not required, or even if the cooling system isprovided, a level of cooling performance thereof can be made lower ascompared to a conventional system.

Therefore, when cooling of the refrigerant of the superconducting cableis also taken into consideration, the superconducting cable line of thepresent invention having the construction as described above canincrease a coefficient of performance as compared to a conventional linebecause energy for cooling the refrigerant can be substantially reducedby reducing the heat intrusion into the cable, as described above. Inparticular, reduction of the heat intrusion is extremely effective forincreasing the coefficient of performance when the line of the presentinvention is used as a line for DC transmission in which heat (aconductor loss) is hardly generated with passage of a current, since theheat intrusion becomes a main cause of an energy loss in this situation.

In addition, in the line of the present invention, energy for coolingliquid hydrogen is also significantly decreased by utilizing therefrigerant of the superconducting cable as an object of a heat exchangefor cooling liquid hydrogen. Therefore, the present invention cantotally reduce energy for cooling the refrigerant of the superconductingcable and energy for cooling liquid hydrogen to substantially increasethe coefficient of performance.

Furthermore, when a superconducting cable including a cable core havingan electrical insulation layer subjected to ρ grading is utilized in theline of the present invention, the line can have a good DC withstandvoltage property and can be suitable for DC transmission. In addition,when a superconducting cable including a cable core having an electricalinsulation layer subjected to ρ grading and provided to have a higher εvalue in a portion near a superconducting conductor layer is utilized inthe line of the present invention, an Imp. withstand voltage propertycan also be increased in addition to an increase in the DC withstandvoltage property as described above. In particular, the line of thepresent invention can also have good AC electric characteristics whenthe electrical insulation layer is formed to have an ε value increasedtoward an inner periphery side and decreased toward an outer peripheryside. Therefore, the superconducting cable line of the present inventioncan be suitably utilized for each of DC transmission and ACtransmission. In addition, when the superconducting cable including thecable core having the electrical insulation layer subjected to ρ gradingand ε grading is utilized as the line of the present invention and aterminal structure formed in an end portion of the line has aconstruction having a variable cross-sectional area of a conductor of anextraction conductor portion arranged between the superconducting cableand a conductive portion on an ordinary temperature side, the line ofthe present invention can be suitably utilized in a transient period inwhich a transmission system is changed from an AC system to a DC system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a construction of asuperconducting cable line of the present invention.

FIG. 2 is a schematic cross-sectional view of a construction of aportion near a superconducting cable in the superconducting cable lineof the present invention.

FIG. 3 is a schematic view of a construction in which thesuperconducting cable line of the present invention is constructed.

FIG. 4 is a schematic view of a construction of the superconductingcable line of the present invention including a transportation pipe forliquid hydrogen, the superconducting cable and a heat exchanger spacerinside a heat insulation pipe for a fluid, which is a schematiccross-sectional view of a construction of a portion near the cable.

FIG. 5 is a schematic view of a construction of a terminal structureformed in an end portion of the superconducting cable line of thepresent invention using a three-core type superconducting cable in asituation of an AC transmission line.

FIG. 6 is a schematic view of a construction of a terminal structureformed in an end portion of the superconducting cable line of thepresent invention using a three-core type superconducting cable in asituation of a DC transmission line.

FIG. 7 is a cross-sectional view of a three-core type superconductingcable for three-phase AC transmission.

FIG. 8 is a cross-sectional view of each cable core.

DESCRIPTION OF THE REFERENCE SIGNS

1: liquid hydrogen, 2: heat insulation pipe for a fluid, 2 a: externalpipe, 2 b: internal pipe, 3: transportation pipe, 4: heat exchangerspacer, 10: superconducting cable, 11: heat insulation pipe for a cable,11 a: external pipe, 11 b: internal pipe, 12: cable core, 13: space, 14:superconducting conductor layer, 15: external superconducting layer, 16:transportation pipeline, 20: hydrogen station, 21: tank, 22:transportation pipeline, 30: heat exchange means, 31: passage, 32:expansion valve, 33: compressor, 34: heat insulation case, 40:extraction conductor portion, 41: low temperature-side conductorportion, 41 a: low temperature-side seal portion, 42: ordinarytemperature-side conductor portion, 42 a: ordinary temperature-side sealportion, 43: lead, 44: ground line, 50: end connection box, 51, 52:refrigerant bath, 53: vacuum insulation bath, 53 a: extensible portion,60: bushing, 61: extraction conductor portion, 62: hollow porcelain, 63:epoxy unit, 70: short-circuited portion, 100: superconducting cable forthree-phase AC transmission, 101: heat insulation pipe, 101 a: externalpipe, 101 b: internal pipe, 102: cable core, 103: space, 104:corrosion-proof layer, 200: former, 201: superconducting conductorlayer, 202: electrical insulation layer, 203: superconducting shieldlayer, 204: protection layer.

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described.

EXAMPLE 1

FIG. 1 is a schematic cross-sectional view of a construction of asuperconducting cable line of the present invention. FIG. 2 is aschematic cross-sectional view of a construction of a portion near asuperconducting cable in the superconducting cable line of the presentinvention. FIG. 3 is a schematic view of a construction in which thesuperconducting cable line of the present invention is constructed. Thesame characters in the drawings indicate the same portions. Thesuperconducting cable line of the present invention includes a heatinsulation pipe for a fluid 2 transporting liquid hydrogen 1, asuperconducting cable 10 housed in heat insulation pipe for a fluid 2,and heat exchange means 30 for adjusting a temperature of liquidhydrogen 1 and a temperature of a refrigerant of the cable.

Superconducting cable 10 utilized in this example has a construction inwhich three cable cores 12 are stranded and housed in a heat insulationpipe for a cable 11, which construction is basically similar to that ofa superconducting cable shown in FIG. 7. Each cable core 12 includes,from a center portion thereof, a former, a superconducting conductorlayer, an electrical insulation layer, an external superconductinglayer, and a protection layer. Each of the superconducting conductorlayer and the external superconducting layer was formed with aBi2223-based superconducting tape line (an Ag—Mn sheath line). Thesuperconducting conductor layer and the external superconducting layerwere formed by spirally winding the superconducting tape line on anouter periphery of the former and on an outer periphery of theelectrical insulation layer, respectively. A plurality of strandedcopper lines were used as the former. A cushion layer was formed betweenthe former and the superconducting conductor layer with insulatingpaper. The electrical insulation layer was constructed by windingsemisynthetic insulating paper (PPLP: a trademark of Sumitomo ElectricIndustries, Ltd.) on an outer periphery of the superconducting conductorlayer. The protection layer was provided by winding kraft paper on anouter periphery of the external superconducting layer. An internalsemi-conducting layer and an external semi-conducting layer may berespectively provided on an inner periphery side and an outer peripheryside (below the external superconducting layer) of the electricalinsulation layer. Three cable cores 12 as such are prepared, strandedwith slack to have an allowance for thermal contraction, and housed inheat insulation pipe for a cable 11. In this example, an SUS corrugatedpipe was used to form heat insulation pipe for a cable 11, in which aheat insulating material (not shown) having a multilayer structure wasarranged between a double pipe formed with an external pipe 11 a and aninternal pipe 11 b, and air between external pipe 11 a and internal pipe11 b was evacuated to attain a prescribed degree of vacuum to form avacuum multilayer insulation construction. Space 13 enclosed with aninner periphery of internal pipe 11 b and outer peripheries of threecable cores 12 becomes a passage of the refrigerant. The refrigerant forcooling the superconducting conductor layer and the externalsuperconducting layer is circulated in this passage using a pump or thelike. In this example, liquid nitrogen (about 77 K) was utilized as therefrigerant. A pipeline 16 is connected to heat insulation pipe for acable 11 of superconducting cable 10 for performing circulationtransportation of the refrigerant in which, for example, the refrigerantis ejected from heat insulation pipe 11 to a side of heat exchange means30 and the refrigerant flows from the side of heat exchange means 30into heat insulation pipe 11. A pump, which is not shown, is arranged ona portion of pipeline 16 to circulate the refrigerant.

Superconducting cable 10 having a construction as described above ishoused in heat insulation pipe for a fluid 2. Heat insulation pipe for afluid 2 in this example has a construction of a double pipe structureformed with an external pipe 2 a and an internal pipe 2 b, in which aheat insulating material (not shown) is arranged between pipes 2 a, 2 b,and air between the pipes is evacuated. Space enclosed with an innerperiphery of internal pipe 2 b and an outer periphery of superconductingcable 10 becomes a transportation passage for liquid hydrogen 1. Each ofpipes 2 a, 2 b was a welded pipe made of steel, and cable 10 was housedin internal pipe 2 b by arranging superconducting cable 10 on a steelplate for forming internal pipe 2 b and welding both edges of the steelplate. In this example, superconducting cable 10 is arranged in internalpipe 2 b while being immersed in liquid hydrogen. Heat insulation pipefor a fluid 2 in this example was formed to construct a pipeline fortransporting liquid hydrogen from a hydrogen plant (not shown) to eachhydrogen station 20. Each hydrogen station 20 includes a tank 21 storingliquid hydrogen and heat exchange means 30 for exchanging heat betweenliquid hydrogen 1 and the refrigerant of superconducting cable 10. Tank21 is connected to heat insulation pipe for a fluid 2 and stores liquidhydrogen transported through heat insulation pipe for a fluid 2. Inaddition, a pipeline 22 is connected to tank 21 for performingcirculation transportation of liquid hydrogen in which, for example,liquid hydrogen is ejected from tank 21 to a side of heat exchange means30 and liquid hydrogen flows from the side of heat exchange means 30into tank 21. A pump, which is not shown, is included in a portion ofpipeline 22 to circulate liquid hydrogen.

Heat exchange means 30 in this example includes a passage 31 circulatinga heat-exchanging medium such as helium, an expansion valve 32 expandingthe heat-exchanging medium, a compressor 33 compressing the expandedheat-exchanging medium, and a heat insulation case 34 housing theseelements. Pipeline 22 is arranged such that, a portion of pipeline 22for circulation transportation of liquid hydrogen contacts a portion ofpassage 31 which passed through expansion valve 32 so as to cool liquidhydrogen with the expanded heat-exchanging medium. With thisconstruction, liquid hydrogen is cooled near the portion of pipeline 22contacting the portion of passage 31 which passed through expansionvalve 32. Therefore, liquid hydrogen ejected from tank 21 flows throughpipeline 22, is cooled with heat exchange means 30, and returns to tank21. In addition, pipeline 16 is arranged such that, a portion ofpipeline 16 for circulation transportation of the refrigerant (liquidnitrogen) of cable 10 contacts a portion of passage 31 which passedthrough compressor 33 so as to rise a temperature of the refrigerant ofcable 10, which was cooled with liquid hydrogen, with the compressedheat-exchanging medium within a temperature range capable of maintainingthe superconducting state. With this construction, the temperature ofthe refrigerant is risen near the portion of pipeline 16 contacting theportion of passage 31 which passed through compressor 33. Therefore, therefrigerant ejected from heat insulation pipe for a cable 11 flowsthrough pipeline 16, has the temperature risen with heat exchange means30, and returns to heat insulation pipe 11.

The superconducting cable housed in the heat insulation pipe for a fluidhas an outer periphery covered with cryogenic liquid hydrogen, and has adouble heat insulation structure formed with the heat insulation pipe ofthe cable itself and the heat insulation pipe for liquid hydrogen. Withthis construction, the line of the present invention can substantiallyreduce heat intrusion from the outside into the superconducting cable.In addition, since the outer periphery of the superconducting cable iscovered with cryogenic liquid hydrogen, heat from liquid hydrogen isconducted to the cable and the refrigerant of the cable is cooled.Therefore, a cooling system for cooling the refrigerant of thesuperconducting cable may not be necessary. As a result, energy forcooling the refrigerant of the superconducting cable can be reduced anda coefficient of performance can be increased by constructing thesuperconducting cable line of the present invention.

Furthermore, since the line of the present invention includes the heatexchange means for exchanging heat between the refrigerant of thesuperconducting cable and liquid hydrogen to concurrently performheating of the refrigerant and cooling of liquid hydrogen, a temperaturedifference between objects of a heat exchange can be decreased andenergy for cooling liquid hydrogen can be reduced with the heat exchangemeans. In addition, with the heat exchange means included in the line ofthe present invention, heat associated with cooling of liquid hydrogencan be utilized to rise the temperature of the refrigerant of thesuperconducting cable which was excessively cooled because of beinghoused in the heat insulation pipe for a fluid. Therefore, utilizing theheat exchange means constructed to exchange heat between liquid hydrogenand the refrigerant of the superconducting cable, the line of thepresent invention can adjust a temperature of liquid hydrogen to anappropriate temperature and can also adjust a temperature of therefrigerant of the cable to an appropriate temperature. As a result,energy for cooling the refrigerant of the superconducting cable andenergy for cooling liquid hydrogen can both be reduced by constructingthe superconducting cable line of the present invention.

It is to be noted that, though a construction shown in this example hasa whole length in a longitudinal direction of the superconducting cablehoused in the heat insulation pipe for a fluid, only a portion of thecable may be housed in the heat insulation pipe for a fluid. In the lineof the present invention, an effect of reduction of heat intrusion maybe decreased and adjustment of the temperature of the refrigerant of thesuperconducting cable with the heat exchange means may become difficultwhen only a small portion of the superconducting cable is housed in theheat insulation pipe for a fluid. Therefore, in the line of the presentinvention, a sufficient portion of the superconducting cable is housedin the heat insulation pipe for a fluid to allow adjustment of thetemperature of the refrigerant of the superconducting cable with theheat exchange means.

EXAMPLE 2

Though the superconducting cable was immersed in liquid hydrogen inexample 1 described above, the superconducting cable may be housed inthe heat insulation pipe for a fluid without being immersed in liquidhydrogen. As an example, a transportation passage for liquid hydrogenmay be separately provided in the heat insulation pipe for a fluid. FIG.4 is a schematic view of a construction of the superconducting cableline of the present invention including a transportation pipe for liquidhydrogen and a heat exchanger spacer inside the heat insulation pipe fora fluid, which is a schematic cross-sectional view of a construction ofa portion near the cable. This superconducting cable line has aconstruction including a separate transportation pipe 3 for liquidhydrogen in internal pipe 2 b of heat insulation pipe for a fluid 2. Aheat exchanger spacer 4 having high heat conductivity is arranged inspace enclosed with an inner periphery of internal pipe 2 b, an outerperiphery of transportation pipe 3 and an outer periphery ofsuperconducting cable 10. With this construction, superconducting cable10 has a double heat insulation structure formed with heat insulationpipe for a fluid 2 and heat insulation pipe 11 of cable 10 itself (seeFIGS. 1, 2) as in example 1, and therefore heat intrusion from theoutside into the cable can be reduced. In addition, since heat fromliquid hydrogen is conducted to superconducting cable 10 via heatexchanger spacer 4, cable 10 can also be cooled with liquid hydrogen 1.Furthermore, since transportation pipe 3 is included to physicallyseparate superconducting cable 10 from liquid hydrogen 1, a problem suchas firing of liquid hydrogen 1 can be prevented when an accident such asa short circuit of cable 10 occurs and a spark is generated. In thisexample, the heat exchanger spacer was formed by winding aluminum.

The superconducting cable line of the present invention shown in each ofexamples 1 and 2 described above can be utilized for either of DCtransmission and AC transmission. In a situation of the DC transmission,when the superconducting cable including the cable core having theelectrical insulation layer subjected to ρ grading to have a lowresistivity on an inner periphery side and a high resistivity on anouter periphery side is utilized, a DC electric field distribution in athickness direction of the electrical insulation layer can be smoothedand a DC withstand voltage property can be increased. The resistivitycan be varied using PPLP (trademark) having various ratios k. Theresistivity tends to increase as ratio k increases. In addition, when ahigh ε layer is provided in the electrical insulation layer near thesuperconducting conductor layer, an Imp. withstand voltage property canbe increased in addition to an increase in the DC withstand voltageproperty. The high ε layer may be formed using, for example, PPLP(trademark) having a low ratio k. In this situation, the high ε layeralso becomes a low ρ layer. Furthermore, the superconducting cableincluding the cable core having the electrical insulation layersubjected to the ρ grading and also formed to have permittivity εincreased toward the inner periphery side and decreased toward the outerperiphery side also has good AC characteristics. Therefore, the line ofthe present invention utilizing the cable as such can also be suitablyutilized for the AC transmission. As an example, the electricalinsulation layer may be provided using PPLP (trademark) having variousratios k as indicated below to have three different resistivities andpermittivities. The following three layers may be successively providedfrom the inner periphery side (X and Y are constants).

Low ρ layer: ratio k=60%, resistivity ρ (20° C.)=X [Ω·m], permittivityε=Y

Intermediate ρ layer: ratio k=70%, resistivity ρ (20° C.)=about 1.2X[Ω·cm], permittivity ε=about 0.95Y

High ρ layer: ratio k=80%, resistivity ρ (20° C.)=about 1.4X [Ω·cm],permittivity ε=about 0.9Y

When monopolar transmission is performed with the line of the presentinvention using the superconducting cable subjected to the ρ grading andthe ε grading, two cores out of three cable cores 12 (see FIG. 2) may beused as auxiliary cores, the superconducting conductor layer of one coremay be used as a go conductor and the external superconducting layer ofthe core may be used as a return conductor. Alternatively, thesuperconducting conductor layer of each core may be used as the goconductor and the external superconducting layer of each core may beused as the return conductor to construct a three-line monopolartransmission line. On the other hand, when bipolar transmission isperformed, one core out of three cores may be used as an auxiliary core,the superconducting conductor layer of one core may be used as apositive electrode line, the superconducting conductor layer of anothercore may be used as a negative electrode line, and the externalsuperconducting layers of both cores may be used as neutral conductorlayers.

The line of the present invention using the superconducting cablesubjected to the ρ grading and the ε grading and including a terminalstructure as described below can readily perform the DC transmissionsuch as monopolar transmission or bipolar transmission after the ACtransmission, or the AC transmission after the DC transmission. Each ofFIGS. 5 and 6 is a schematic view of a construction of a terminalstructure having a removable extraction conductor portion, which isformed in an end portion of the superconducting cable line of thepresent invention using a three-core type superconducting cable. FIG. 5indicates a situation of an AC transmission line, and FIG. 6 indicates asituation of a DC transmission line. Though two cable cores 12 are onlyshown in each of FIGS. 5 and 6, there actually are three cores.

The terminal structure includes an end portion of cable core 12extending from an end portion of superconducting cable 10, extractionconductor portions 40, 61 connected to a conductive portion (not shown)on an ordinary temperature side, a connection portion electricallyconnecting the end portion of core 12 with extraction conductor portion40 and the end portion of core 12 with extraction conductor portion 61,and an end connection box 50 housing the end portion of core 12, endportions of extraction conductor portions 40, 61 on a side connected tothe core, and the connection portion. End connection box 50 includes arefrigerant bath 51 filled with a refrigerant for coolingsuperconducting conductor layer 14, into which superconducting conductorlayer 14 exposed by step stripping of the end portion of core 12 isintroduced, a refrigerant bath 52 filled with a refrigerant for coolingan external superconducting layer 15, into which externalsuperconducting layer 15 also exposed by step stripping is introduced,and a vacuum insulation bath 53 arranged on outer peripheries ofrefrigerant baths 51, 52. Extraction conductor portion 61, which isembedded in bushing 60 arranged between the conductive portion on theordinary temperature side and superconducting conductor layer 14, isconnected to superconducting conductor layer 14 via a joint (aconnection portion) to allow transmission and reception of power betweensuperconducting cable 10 and the conductive portion on the ordinarytemperature side. A side (an ordinary temperature side) of bushing 60connected to the conductive portion on the ordinary temperature sideprojects from vacuum insulation bath 53 and is housed in hollowporcelain 62 provided to project from vacuum insulation bath 53.

On the other hand, external superconducting layer 15 is connected via ashort-circuited portion 70 (a connection portion) described below toextraction conductor portion 40 arranged between the conductive portionon the ordinary temperature side and external superconducting layer 15to allow transmission and reception of power between superconductingcable 10 and the conductive portion on the ordinary temperature side.Extraction conductor portion 40 is formed with a low temperature-sideconductor portion 41 connected to short-circuited portion 70 and anordinary temperature-side conductor portion 42 arranged on the ordinarytemperature side which is removable from low temperature-side conductorportion 41. In this example, ordinary temperature-side conductor portion42 is formed in a rod-like shape having a prescribed cross-sectionalarea, and low temperature-side conductor portion 41 is formed in acylindrical shape into which the rod-like ordinary temperature-sideconductor portion 42 can be fitted. When ordinary temperature-sideconductor portion 42 is inserted into low temperature-side conductorportion 41, the portions 41 and 42 are electrically connected to eachother to allow transmission and reception of power between the lowtemperature side and the ordinary temperature-side, and the portions 41and 42 are brought out of conduction when ordinary temperature-sideconductor portion 42 is removed from low temperature-side conductorportion 41. A plurality of extraction conductor portions 40 as such areincluded in the terminal structure. Low temperature-side conductorportion 41 is fixed on refrigerant bath 52 and has one end electricallyconnected to short-circuited portion 70 and the other end arranged toenter vacuum insulation bath 53. A low temperature-side seal portion 41a made of FRP is provided on an outer periphery of a fixing portion oflow temperature-side conductor portion 41 to avoid leaking of therefrigerant, short-circuiting of refrigerant bath 52 and conductorportion 41, and the like. Ordinary temperature-side conductor portion 42is fixed on vacuum insulation bath 53 and has one end arranged in vacuuminsulation bath 53 and the other end arranged to be exposed to theoutside of an ordinary temperature. An ordinary temperature-side sealportion 42 a made of FRP is provided on an outer periphery of a fixingportion of ordinary temperature-side conductor portion 42 to allowreduction of heat intrusion and to avoid short-circuiting of vacuuminsulation bath 53 and conductor portion 42 and the like. In addition,an extensible portion 53 a formed with a corrugated pipe is provided onvacuum insulation bath 53 near the fixing portion of ordinarytemperature-side conductor portion 42 to maintain a vacuum state ofvacuum insulation bath 53 during attachment and removal of extractionconductor portion 40. It is to be noted that, external superconductinglayer 15 of each of three cores 12 is short-circuited in short-circuitedportion 70. In addition, a lead 43 connected to an external apparatus orthe like, or a ground line 44 is attached to an end portion on theordinary temperature side of ordinary temperature-side conductor portion42. An epoxy unit 63 is arranged on an outer periphery of a portion ofsuperconducting conductor layer 14 which is arranged near a portionbetween refrigerant baths 51, 52.

When the superconducting cable line of the present invention includingthe terminal structure having a construction as described above isutilized as, for example, a three-phase AC line, extraction conductorportion 40 connected to external superconducting layer 15 should have across-sectional area of the conductor required to obtain a voltage toground. Therefore, as shown in FIG. 5, while low temperature-sideconductor portion 41 and ordinary temperature-side conductor portion 42of extraction conductor portion 40 needed are connected to each other,low temperature-side conductor portion 41 and ordinary temperature-sideconductor portion 42 of extraction conductor portion 40 not needed areseparated from each other to obtain a required cross-sectional area ofthe conductor. In this example, ground line 44 for grounding isconnected to the end portion on the ordinary temperature side ofordinary temperature-side conductor portion 42 of extraction conductorportion 40 which is connected.

On the other hand, when a change from the three-phase AC transmission asshown in FIG. 5 to the DC transmission is requested, a currentequivalent to that for superconducting conductor layer 14 flows throughexternal superconducting layer 15. That is, the current flowing throughexternal superconducting layer 15 is increased and a current flowingthrough extraction conductor portion 40 is also increased as compared tothose in the situation of the AC transmission shown in FIG. 5.Therefore, as shown in FIG. 6, low temperature-side conductor portion 41and ordinary temperature-side conductor portion 42 of extractionconductor portion 40 which were separated during the AC transmission areconnected to each other to ensure a sufficient cross-sectional area ofthe conductor for passing a required amount of current. In this example,lead 43 for grounding is connected to the end portion on the ordinarytemperature side of ordinary temperature-side conductor portion 42 ofextraction conductor portion 40 which is connected. Reversely, when achange from the DC transmission as shown in FIG. 6 to the ACtransmission is requested, one of extraction conductor portions 40 whichwas brought into conduction during the DC transmission is separated tobring out of conduction.

INDUSTRIAL APPLICABILITY

A superconducting cable line of the present invention is suitablyutilized as a line for performing power transmission to various powerapparatuses. The superconducting cable line of the present invention maybe constructed by, for example, housing a superconducting cable in apipeline transporting liquid hydrogen and arranging heat exchange meansin a hydrogen station connected to the pipeline. In this situation, theline of the present invention may be utilized as a power supply line fora power apparatus inside the hydrogen station or as a power supply linefor an arbitrary power apparatus, which draws power as required from aheat insulation pipe for a fluid. In addition, since the cable line ofthe present invention can be constructed during construction of atransportation passage for liquid hydrogen or the hydrogen station,workability for laying is increased.

1. A superconducting cable line, comprising: a heat insulation pipe fora fluid for transporting liquid hydrogen; a superconducting cable housedin said heat insulation pipe for a fluid for cooling a superconductingportion with a refrigerant having a temperature higher than that of saidliquid hydrogen; and heat exchange means for cooling said liquidhydrogen and rising a temperature of the refrigerant of the cable cooledwith said liquid hydrogen.
 2. The superconducting cable line accordingto claim 1, wherein said superconducting cable is immersed in saidliquid hydrogen.
 3. The superconducting cable line according to claim 1,wherein a region inside said heat insulation pipe for a fluid is dividedinto a transportation region for transporting said liquid hydrogen and aregion for arranging said superconducting cable therein.
 4. Thesuperconducting cable line according to claim 1, wherein the refrigerantof said superconducting cable is liquid nitrogen.
 5. The superconductingcable line according to claim 1, wherein said superconducting cableincludes a superconducting conductor layer and an electrical insulationlayer provided on an outer periphery of said superconducting conductorlayer, and said electrical insulation layer is subjected to ρ gradingfor attaining a low resistivity on an inner periphery side of saidelectrical insulation layer and a high resistivity on an outer peripheryside to smooth a DC electric field distribution in a diameter directionthereof.
 6. The superconducting cable line according to claim 5, whereinsaid electrical insulation layer has a high ε layer provided near saidsuperconducting conductor layer and having a permittivity higher thanthat in another portion.
 7. The superconducting cable line according toclaim 5, wherein said electrical insulation layer is constructed to havea permittivity ε increased toward the inner periphery side and decreasedtoward the outer periphery side.